1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer or a copying machine, and, in particular, to a sheet conveying apparatus equipped with a skew feed correcting device for correcting skew feed of a sheet.
2. Related Background Art
Generally, an image forming apparatus, such as a printer or a copying machine, is equipped with a sheet conveying apparatus for conveying a sheet as a transfer material. In the sheet conveying apparatus, a phenomenon called skew feed may occur during conveyance due to misalignment of the conveying roller, distortion of a sheet guide, etc. that are attributable to inadequacy in the assembly precision of the components. If image formation is conducted with skew feed neglected, the resultant image is naturally distorted with respect to the sheet. In particular, in an image forming apparatus with a two-side printing function, there may be involved a marked deviation between the transfer image on the first side and the transfer image on the second side.
In view of this, many image forming apparatuses are equipped with a skew feed correcting device for correcting skew feed of a sheet. The known skew feed correcting devices may be roughly classified into two types.
First, a sheet correction system utilizing registration rollers as a reference as disclosed, for example, in JP H01-053886 A, will be described. A pair of registration rollers are arranged so as to extend in a direction substantially perpendicular to the sheet conveying direction. On the upstream side of the registration roller pair, there is arranged a blocked sheet guide. A sheet is caused to abut the registration roller pair, with the rotation of the registration rollers at rest, and the conveyance of the sheet is continued, whereby a loop is formed in the sheet. As a result, the leading edge of the sheet is in conformity with the nip line of the registration rollers, thus making it possible to correct skew feed.
Then, the registration roller pair conveys the sheet toward a transfer means in synchronism with a toner image formed by an image forming means. That is, in this conventional construction, the registration roller pair effects positioning on the sheet with respect to the two directions of the sheet rotating direction (skew feed) and the sheet conveying direction.
Next, a side-registration-type skew feed correcting device as disclosed, for example, in JP H11-189355 A, will be described. In a part of a blocked sheet guide, there are provided a reference wall parallel to the sheet conveying direction and a diagonal feed roller whose rotating surface is inclined toward the reference wall. Sheet conveyance is effected while causing a sheet side edge portion to move along the reference wall, whereby skew feed correction is effected.
The system utilizing the registration rollers as a reference is adopted in many image forming apparatuses of the type which use the sheet center as a reference and in which the sheet conveyance is divided in the middle portion of the conveying path, whereas the side-registration-type skew feed correction system is adopted in many image forming apparatuses of the sheet-side reference type, in which a reference is provided on one side of the apparatus and in which a side edge portion of a sheet is aligned with this reference.
The above-described conventional skew feed correcting devices, however, have the following problems.
First, the problem in the registration-roller-type skew feed correcting device will be discussed. This skew feed correcting device, which corrects skew feed by using the sheet leading edge as a reference, is capable of correcting skew feed of the trailing edge of a sheet, but is incapable of correcting deviation in the sheet width direction (the direction orthogonal to the conveying direction). Further, since the sheet is caused to abut the registration roller pair, it is necessary to effect temporary stopping, which makes it difficult to achieve an improvement in productivity. Further, when effecting correction on a thick-fore-edge sheet, such as the cover of a booklet, it may occur that a loop cannot be formed due to the high rigidity of the sheet. Further, in some cases, the sheet may penetrate the nip of the registration rollers, thus making it impossible to perform the alignment itself. Further, in an image forming apparatus with a two-side printing function, when effecting switch-back reversal on the sheet, the leading edge of the first surface is replaced by that of the second surface, so that a deviation in parallelism between the leading and trailing edges of the sheet manifests itself as an image deviation.
Next, the problem in the side-registration-type skew feed correcting device will be discussed. As is characteristic of a sheet-side-reference type conveying system, the sheet holding position is deviated to the side-reference side with respect to the center of the conveying path (sheet side reference). Normally, the conveying roller is rotatably supported by a side plate through the intermediation of bearings, and receives a pressure for holding and conveyance from an opposing pressurizing roller. When the pressure is applied from the pressurizing roller to the conveying roller, deflection is generated in the conveying roller, and the pressure tends to be relatively high at the ends of the conveying roller and relatively low in the middle portion thereof. Thus, in a sheet-side-reference type conveying system, the holding pressure is relatively high on one side of the sheet (an end portion of the conveying path), and relatively low on the other side thereof (the side near the center of the conveying path). A high holding pressure results in an increase in conveying speed, which leads to skew feed.
Further, due to the irregularity in holding pressure, the roller undergoes uneven wear, which is also liable to cause skew feed in the conveying path. In particular, when the uneven wear progresses as a result of long-term use, it is to be expected that, sooner or later, the skew feed amount will increase beyond the permissible amount for the skew feed correcting device. On the other hand, provision of a pressure adjustment mechanism for making the holding pressure in the width direction of the conveying roller uniform would complicate the device construction and adjusting operation and lead to an increase in production cost, which is not desirable.